Why could Saul's troops find no swords or spears on the day of battle?

Answer: Israel had no blacksmiths

The Philistines allowed no blacksmiths in Israel so the Hebrews could not make weapons. Only Saul and Jonathan had them.

“And no blacksmith could be found in all the land of Israel, because the Philistines had said, “The Hebrews must not be allowed to make swords or spears.””— 1 Samuel 13:19 (BSB)